Improvement of quality of oocytes collected in the autumn by enhanced removal of impaired follicles from previously heat-stressed cows

The fertility of dairy cows decreases during the summer and remains low dur ing the cooler autumn although the animals are no longer under heat stress. The aim of this study was to characterize a delayed effect of summer heat stress on oocyte quality in the autumn and to improve oocyte quality by enh anced removal of follicles damaged during the previous summer. Lactating co ws (n = 16) were subjected to heat stress during the summer. In autumn, ova rian follicles (3-7 mm in diameter) were aspirated by an ultrasound-guided procedure during four consecutive oestrous cycles. Follicles were aspirated from control cows on day 4 and from treated cows on days 4, 7, 11 and 15 o f each oestrous cycle. All cows received PGF(2 alpha) and GnRH injections o n days 19 and 21, respectively, and maintained cyclicity, as indicated by p lasma progesterone concentrations. On day 4 of each cycle, the oocytes reco vered were examined morphologically, matured and activated in vitro, and cu ltured for 8 days. In cycle I (early October) both groups showed low percen tages of grade I oocytes, cleavage, four- and eight-cell embryos, morulae a nd parthenogenetic blastocysts. Subsequently, the number of grade I oocytes increased earlier (cycle 2) in treated than in control cows (cycle 3; P < 0.05). The cleavage rate in the control group remained relatively low throu ghout (32-58%), whereas in the treated group it increased from 40% (cycle 1 ) to 75 % (cycles 3 and 4; P < 0.05). The number at each stage of embryo de velopment increased slightly but remained low throughout in the control gro up, whereas in the treated group significant (P < 0.05) increases of all st ages were observed in cycles 3 and 4. The results show a delayed effect of summer heat stress on oocyte quality and embryo development in the autumn. Enhanced removal of the impaired cohort of follicles led to earlier emergen ce of healthy follicles and high quality oocytes in the autumn.